A Liberal MP has released a rival bill that would legalise same-sex marriage but dramatically roll back and override anti-discrimination provisions to allow "conscientious objectors" to boycott gay weddings.

The bill, sponsored by Liberal senator James Paterson - a "yes" supporter and a vocal proponent of individual liberty - clashes with an existing proposal from his colleague Dean Smith, which has already been backed by Labor and some Coalition ministers.

Its release now, ahead of the results of the postal survey on Wednesday, is an attempt by conservatives to shape the debate about how to legislate for same-sex marriage in the widely-expected advent of a "yes" vote.

The Paterson plan would allow any person or business to refuse to co-operate with the staging of a same-sex wedding, protecting them from civil litigation under anti-discrimination laws.

It would override existing state and territory anti-discrimination laws, stating plainly that when the two come into conflict, the federal law would prevail.

And it goes further than just protecting beliefs about the nature of marriage, establishing what Senator Paterson calls a "relevant belief" about homosexuality itself.

Anyone who holds and expresses a belief that same-sex relationships are unholy or immoral would also be protected from anti-discrimination laws by the Paterson bill, as would anyone who believes "the normative state of gender is binary".

Furthermore, the proposal tells celebrants it is within their power to decide if a person is "a man or a woman", and allows them to ignore the legal status of an intersex or transgender person if they believe the person isn't really male or female.

And, in a key bone for conservatives, the bill does not actually remove the definition of marriage as between "a man and a woman", inserted under John Howard's leadership in 2004. Instead, it adds a second clause declaring marriage can also be "the union of two people".

Senator Paterson told Fairfax Media he was proud to have voted "yes" in the postal survey, but: "30 to 40 per cent of Australians will vote 'no' and I don't want to see their rights and freedoms diminished at all".

He said the protection of "relevant beliefs" about homosexuality was necessary because people's beliefs about marriage were often underpinned by beliefs about same-sex relationships more broadly.

"They have those beliefs for a reason," he said, and without such a clause "we wouldn't really have free speech in this country, and I think that would be a real problem".

Senator Paterson said keeping the old definition of marriage as between a man and a woman would provide "an extra layer of protection" because it was "less likely someone would be discriminated against if that definition remains in the law".

He also clarified the exemptions would apply only to goods and services "directly connected" to a wedding, such as venue hire, audio/visual equipment, photography, cakes and flowers.

A person wishing to buy a gift for a same-sex wedding, or catch a taxi to attend one, could not be refused service under the proposed law.

"It's not about the person, it's about the event. You cannot decline to supply a person based on their own characteristics [but] you can decline to participate in their wedding," Senator Paterson said.

The Victorian senator, a former policy fellow with the Institute of Public Affairs, said he was initially reluctant to put forward an alternative same-sex marriage bill, but did so because no-one else was acting.

He said he consulted widely, including "yes" and "no" supporters in the Parliament, and hoped the bill would command "broad support" in the Coalition party room.

The Equality Campaign slammed the Paterson proposal as regressive and unfair.

"It's a rebuke to the Australian people," said campaign director Tiernan Brady. "It literally is the opposite of what the people will have voted for.

"We're not going back to a time when we have signs on windows saying certain people can't apply or certain people won't be served. That is precisely what the bill does."

Whether the Paterson proposal will ever be debated in the Coalition party room is unclear. Prime Minister Malcolm Turnbull and several cabinet ministers - including conservative Finance Minister Mathias Cormann on Monday - have pointed to Senator Smith's bill as their preferred legislation.

Senator Cormann told the ABC he would want amendments to Senator Smith's bill, but it had been through a Senate committee process and was "a good starting position" for negotiations.

Senator Smith said he would introduce his private member's bill as soon as this week if a "yes" vote is announced by the Australian Bureau of Statistics on Wednesday.